1. Field of the Invention
This invention relates to an automatic controller of electrification of magnetic toner, and more particularly to an automatic controller which maintains a predetermined level of electrification for developer containing magnetic toner particles which electrification renders developing capacity to the developer. The automatic controller of the invention is for a developing unit having a magnet roll means to magnetically hold and carry developer containing electrically-insulated magnetic toner particles toward a latent image to be developed and a means to electrify the magnetic toner particles being carried by the magnet roll means.
2. Description of the Prior Art
The art of using a magnet roll means for applying magnetic toner containing enclosed magnetic particles to a surface to be developed has been known, for instance by the disclosure in U.S. Pat. No. 3,909,258. It is also known that there are two developing methods using magnetic toner; namely, (1) a method in which electrically-conductive magnetic toner particles are provided with electric charge of the opposite polarity to that of the electrostatic latent image by using the conductivity thereof, i.e., through a charging circuit formed from the magnetic toner particle, whereby developing capacity is rendered to the magnetic toner; and (2) a method in which magnetic toner particles having a high electric insulation are provided with electric charge of the opposite polarity to that of the electrostatic latent image by various means, whereby developing capacity is rendered to the magnetic toner.
The latter method of using the magnetic toner particles with a high electric insulation has an advantage of stably holding the electric charge because of the insulation of the toner particles, so that this method is particularly suitable to those electrophotographic devices which produce a final picture by transferring a toner image developed on a photosensitive member to an image-receiving paper.
Various improvements have been proposed in the electrifying process of the magnetic toner particles, which process is applicable to developer containing magnetic toner particles with a high electric insulation and to developing units of electrophotographic devices using the transfer of toner images. For instance, (1) in the case of developer exclusively consisting of highly insulating magnetic toner particles of homogeneous inner construction, electric charge is applied to the magnetic toner particles either (i) by triboelectricity between such toner particles and the sleeve of a magnet roll means, which sleeve is insulated or coated with certain substance selected on the basis of its position on the triboelectric series, or (ii) by means of a corona electrifier on an electrode for injecting electric charge; (2) in the case of magnetic toner made of inner magnetic material and outer insulating material, electric charge is applied to the particles by triboelectric effect due to contact of adjacent toner particles; and (3) in the case of magnetic toner particles to be electrified by triboelectricity, the magnetic toner particles are mixed with magnetic or non-magnetic particles made of a certain substance whose position on the triboelectric series is different from that of the magnetic toner particles, so that the magnetic toner particles are electrified by the triboelectricity between the two kinds of particles.
The aforesaid process for electrification of the magnetic toner particles of the prior art has a particular shortcoming in that it is difficult to keep the amount of electric charge on the magnetic toner particles at a constant level, so that it has been difficult to uniformly hold the developing capacity and the transferring efficiency of the toner image.